This invention relates generally to pressure sensors and, more particularly to piezoresistive silicon diaphragm sensors.
Piezoresistive silicon diaphragm sensors are widely used for many industrial and medical pressure sensing applications. A pressure sensor is a three-dimensional displacement transducer that uses electronically monitored, geometric deformation of a diaphragm to measure applied pressures. Various means are used to convert the displacement of the transducer into an electrical output, the most common being the use of piezoelectric resistors deposited on the transducer and connected in a Wheatstone bridge circuit.
Typical prior art diaphragms are of the order of 1000 micrometers.times.1000 micrometers in area with a thickness of approximately 5-20 micrometers. These diaphragms are formed by using anisotropic silicon etching techniques and by using a p+-layer as the etch stop or by precisely controlling the etch time. On top of the single crystal silicon layer are four piezoresistive elements formed either by boron diffusion or ion implantation. A third method of forming piezoresistors on the silicon diaphragm sensor is described in J. Binder et al., "Laser-Recrystallized Polysilicon Resistors for Sensing in Integrated Circuit Applications", Sensors and Actuators, 4(1983) 527-536. Binder et al. uses laser-recystallized polycrystalline silicon as piezoresistors on a single crystal silicon diaphragm. Single crystal silicon diaphragm sensors have been popular due to their low cost of manufacturing and due to the excellent mechanical properties of single crystal silicon in terms of low hysteresis and good stability. Drawbacks of these sensors include the lack of an overpressure stop and yield losses in diaphragm etching. A typical prior art single-crystal silicon diaphragm sensor with piezoresistors deposited into the diaphragm is shown in FIG. 1.
An alternative to the single-crystal silicon diaphragm sensor which does include an overpressure stop is described in "Planar Processed Polysilicon Sealed Cavities for Pressure Transducer Arrays," H. Guckel and D. Burns, Proceedings of the IEDM Meeting, December 1984, San Francisco, CA, pp. 223-225. Guckel et al. form a displacement transducer by depositing an oxide post on a silicon substrate, depositing a polycrystalline silicon layer over the oxide post, then laterally etching the oxide to form the cavity between the polysilicon diaphragm and silicon substrate (which acts as the overpressure stop). Guckel et al. describe several methods for sealing the cavity for sealed pressure transducer applications. Guckel et al. do not indicate the optimum technique for data extraction, but suggests that piezoresistive, capacitive, and optical techniques are available.
The Guckel et al. pressure sensor is a significant advance over the silicon diaphragm sensor in that it offers an overpressure stop. The most significant drawback of the Guckel et al. pressure sensor is the lack of performance which is inherent in polysilicon.
Therefore, it is an object of the present invention to provide a pressure sensor with an overpressure stop and having the performance of single-crystal silicon.
It is another object of the present invention to provide a pressure sensor with an overpressure stop that is suitable for applications requiring sensors smaller than 100 micrometers.times.100 micrometers.
Additional objects, advantages, and novel feature of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.